1. Field of the Invention
This invention relates to a process for preparing an aliphatic polyisocyanate by reacting an aliphatic polyamine or its hydrochloride or carbonate with phosgene.
2. Description of the Related Art
Aliphatic polyisocyanates are extremely useful compounds as raw materials for polyurethane materials, polyurea materials and polyisocyanurate materials in industrial fields such as chemical industry, resin industry and paint industry.
Preparation of an aliphatic polyisocyanate by the phosgene process can be classified roughly into the direct process in which the polyisocyanate is obtained by directly reacting a corresponding polyamine and phosgene and the salt-forming process in which a polyamine salt such as a polyamine hydrochloride or carbonate is first obtained from a corresponding polyamine and hydrochloric acid gas or carbon dioxide gas and the polyamine salt is then reacted with phosgene. Whichever process is employed, a carbamoyl chloride is formed as an intermediate. The carbamoyl chloride is then subjected to dehydrochlorination so that the aliphatic polyisocyanate is prepared.
However, these direct and salt-forming processes are both accompanied by problems. The dehydrochlorination of the carbamoyl chloride into the polyisocyanate takes place at a low reaction velocity and in general, requires a high temperature of at least 120.degree. C., usually 130.degree. C. or higher. When the polyisocyanate so formed is exposed to heat for a long time, the polyisocyanate tends to become tarry, resulting in a reduction in the production yield. Further, hydrochloric acid gas which has been formed as a result of decomposition of the carbamoyl chloride reacts the resultant polyisocyanate, whereby the carbamoyl chloride is formed again. The carbamoyl chloride becomes tarry at a far higher rate than the polyisocyanate, so that the yield is reduced further.
As a solution for the above-described problems, a process has been proposed. According to this process, phosgene is charged at an excess rate to increase the rate of formation of a carbamoyl chloride from a corresponding polyamine and at the same time, to purge hydrochloric acid gas, which remains in a reaction system, out of the reaction system with the excess phosgene, whereby the aliphatic polyisocyanate is prepared with the equilibrium always biased toward the polyisocyanate side. This process can lower the concentration of the carbamoyl chloride during the phosgenation reaction and moreover, can increase the rate of formation of the aliphatic polyisocyanate. This has made it possible to suppress the conversion of the polyisocyanate into tar and hence to obtain the target product at a relatively high yield.
Described specifically, the reaction is conducted generally at a temperature raised to at least 120.degree. C. or 130.degree. C. or higher while charging phosgene at an excess rate.
A further process has also been reported. According to this process, an ester solvent is used to reduce reaction byproducts so that an aliphatic polyisocyanate can be obtained at a high yield (see Japanese Patent Laid-Open Nos. 7253 and 204851/1991).
Even when preparation of an aliphatic polyisocyanate is conducted in accordance with these processes, the conversion of the polyisocyanate into tar cannot be avoided completely, thereby necessitating an additional step for the treatment of the tar. Excess phosgene is entirely wasted as a loss and moreover, requires treatment for making it harmless. These processes therefore cannot still be considered as economically satisfactory processes.